Generally speaking this invention relates to coupling of light into a planar waveguide, where preferably surface reliefs are used as in and out-coupling diffractive grating elements. Important applications include diffractive beam expanders for virtual displays, but the invention might also be applied to other optical imaging or illumination devices, where light needs to be coupled into a planar waveguide with high efficiency. Besides display systems, the invention may in its generic form be utilized in other type of optical systems as well, for example in optical couplers or other light modulator devices.
Display technology is one of the key elements in the development of new portable devices, which today typically feature wireless connectivity for voice and data access—and that will include a display for viewing, for example, text, graphics and different types of multimedia. The displays of such portable devices need to be increasingly capable of reproducing high quality still images and also live video. Such devices include, for example, advanced mobile phones and portable Internet appliances.
The display is the dominant (key) element in many portable products—both physically and from the cost perspective. This drives an increased interest towards virtual displays, especially those using microdisplays as the image source. Virtual displays offer increased displayed image size and resolution, yet physically shrink the size of the image source, i.e. the imager itself. In many cases, the smaller the imager, the lower the cost of the system. So, not only do these displays promise to lower system costs, but their physically smaller size will mean less bulky and heavy products and smaller power consumption, that is they will operate longer on the same battery source. Virtual displays based on microdisplays have high pixel densities and provide good image quality.
Virtual displays use imaging optics to magnify the “input” image from an imager in order to create a virtual “output” image for the observer. A virtual image is what one sees when looking in an electronic viewfinder of a digital video camera, for example. The virtual image appears to be larger and floating at some distance from the eye of the user—even though it is created by a small size integrated imager chip acting as the image source. In other words, the user or observer has the illusion of seeing the source image as if he/she stands at a certain distance from a larger display monitor.
Virtual displays, which are kept close to the eyes, can be monocular or biocular. Other type of virtual displays are, for example, Head Up Displays (HUDs), where the imaging optics are located somewhat further away from the eye.
The present invention may be applied to such virtual display systems, in which diffractive grating elements are used as a part of the imaging optics together with a planar waveguide in order to create an enlarged virtual image from a smaller size real image created by an imager, typically by an integrated circuit display chip. Such virtual display devices are already known in the art. For example, patent publication WO 99/52002 discloses optical devices, in which a plurality of holographic optical elements (HOEs), i.e. diffractive grating elements are arranged on a common planar light-transmissive substrate. Such devices may be used for magnifying the exit pupil of the imaging optics creating a virtual image at infinity and further reflecting this virtual image into the eye of an observer. The enlargement of the exit pupil of a virtual display system with a beam-expanding optical configuration, such as with those described in the document WO 99/52002, results in larger eye relief, which makes the virtual display device more convenient to use. A significantly larger eye relief allows to move the display device further away from the immediate vicinity of the observer's eyes. This makes it possible to observe the virtual display in a manner resembling the use of an ordinary display panel reproducing real images. In certain context such displays may be referred to as window displays.
However, prior art solutions where an in-coupling diffractive grating element is used to couple light into a waveguiding substrate suffer from certain significant limitations. In virtual display applications these limitations degrade the quality of the reproduced virtual images, for example, by reducing the contrast and/or brightness of the produced images.
FIG. 1 describes in a simplified cross-sectional view of one possible configuration of a monocular type diffractive beam expander comprising an in-coupling grating IG and an out-coupling grating OG arranged on a transparent and planar substrate S. In this example the in-coupling grating IG is of reflective type, but it is also possible that it could be of transmissive type, i.e. arranged as schematically shown in FIG. 2 on the lower interface IFL of the substrate S, where the in-coming light wave WI first interacts with the waveguiding substrate S.
When used as a part of a virtual display system, the in-coupling grating IG couples the in-coming light wave WI from an imager via suitable front end optics into the substrate S, where the light is diffracted by said grating in FIG. 1 towards right to become a waveguided light wave WG that propagates along the substrate S. For a person skilled in the art it is clear that light might have been diffracted in a similar manner also to the left in order to have a biocular beam expander. After the aforementioned in-coupling, the light travels inside the substrate S based on total internal reflections (TIR) until the out-coupling grating OG couples the light out from the substrate S towards the observer.
It can be shown, that if the width W of the in-coupling grating IG is arranged to be smaller than 2h tan θ, where h is the thickness of the substrate S and θ is the diffraction angle of the light with respect to the normal of the plane of the in-coupling grating IG, then the in-coming light wave WI interacts with said grating IG only once and after that travels forward inside the substrate plate based on TIR until it meets the out-coupling grating OG. In other words, when the width W of the in-coupling grating IG is kept small enough, then the in-coming light wave experiences first diffraction D1 from the in-coupling grating IG, but after the consequent first TIR1 from the lower interface IFL of the substrate S, the next interaction with the upper surface IFU of the substrate S takes place outside the in-coupling grating IG.
A second interaction of the light wave with the in-coupling grating IG is undesirable, because this causes a significant part of the light to reverse its direction and become diffracted back substantially towards the imager and/or the front end optics. Therefore, a part of the light energy is lost and especially in the case of an imaging system, the contrast is decreased and the overall efficiency is significantly reduced.
However, in order to be able to use a smaller f-number in the front end optics and therefore to have higher light gathering power in said optics, it would be highly desirable to have a way around the above described limitations related to the width W of the in-coupling grating IG.